The use of Doppler radar to detect wind patterns characteristic of tornadoes is fairly well established. One version of this radar is manufactured by Enterprise Electronics of Enterprise, Ala., but, according to the manufacturer, such radar is not covered by a patent because the original system was developed at the Government's expense. This technique can detect wind patterns at distances in excess of one hundred miles, but it cannot determine whether or not a tornado has touched down.
Another device used in the remote detection of tornadoes is the sferics detector, based on the correlation between storm severity and the rate of high-frequency electromagnetic emissions, as disclosed in the following U.S. Pat. Nos.: 3,810,137 to Bacon and Bacon, and 4,812,825 to Kennedy and Fredrich. Such a system has been shown capable of detecting 80% of the tornadoes within a 70-km radius, but unfortunately it is characterized by an unacceptably high false alarm rate.
Several devices have been developed for use by the homeowner for short-range tornado detection. The first is a battery-powered tornado alarm designed to sense the rapid drop in atmospheric pressure, which is generally characteristic of tornadoes, as disclosed in the following U.S. Pat. Nos.: 3,594,753 to Elenbans, 3,631,435 to Elenbans, 3,717,861 to Wright, and 4,632,052 to Green. Such a detection device is considered inadequate, because only a few seconds elapse between the alarm and tornado passage. One system is available which includes both an electromagnetic sensor and a pressure sensor as disclosed by U.S. Pat. Nos. 3,646,540 to Cooper. Another device, which is called Final Alert, is designed to detect the characteristic roaring sound transmitted through the atmosphere by a tornado. This device is due to be introduced in the near future but is not disclosed by any patent as of Apr.30, 1993. Notice should be taken that it is not clear whether Final Alert can distinguish between a tornado on the ground and a funnel cloud passing overhead.
The ideal detection system would provide the occupants of the building, where the system is installed, with a timely warning of the presence of any tornadoes which are in contact with the ground.
The considerable kinetic energy contained within a tornado is a generally accepted fact. The magnitude of this energy may vary, depending on the intensity of the tornado. Whenever in contact with the ground, a tornado encounters considerable frictional resistance. In overcoming such resistance and in interacting with the ground through turbulent pressure fluctuations, as depicted in FIG. 1, the tornado transfers a considerable amount of energy into the ground. A significant portion of the energy absorbed by the ground takes the form of seismic waves or vibrations, with the remainder being transformed into thermal energy. Seismic waves are generally classified as either body waves or surface waves, depending on the region through which they propagate. Such waves may also be classified according to the direction of displacement relative to the direction of propagation. Waves whose displacement is parallel to the direction of propagation are classified as longitudinal, compressional, or P (for primary) waves. Waves whose displacement is perpendicular to the direction of propagation are classified as transverse or S (for secondary or shear) waves. S-waves are further divided into SV (for vertical shear) waves and SH (for horizontal shear) waves .
For the invention only surface waves are of interest. Surface waves generally are divided into Love waves, which are SH type waves, and Rayleigh waves, which are a combination of P type and SV type waves. Because of the nature of the earth's surface the Love waves should be insignificant, and thus the Rayleigh waves are the primary seismic signal generated by tornadoes. The P-type Rayleigh wave is also referred to as the horizontal component of the Rayleigh wave, while the SV-type Rayleigh wave is referred to as the vertical component of the Rayleigh wave.
The inventors, based on theoretical considerations, combined with an examination of seismic recordings and eyewitness accounts, initially established the existence of tornado seismic vibrations (TSV) composed of Rayleigh waves, which propagate radially outward from the tornado through the earth's surface layer. TSV represent a unique means of determining when a tornado is in contact with the ground. Because tornadoes, when on the ground, represent a significant potential for loss of life and property, the vibrations noted represent a tornado seismic signal (TSS) which, if detected, can be used to provide a warning of approaching tornadoes. The minimum range for detection is estimated to be one mile.
Because the inventor's research indicated no system for accomplishing the object of the invention was then available, they set out to develop a system for performing this object. The inventors wanted tc utilize the seismic vibrations produced by a tornado as the warning signal. They therefore included a velocity transducer or geophone as part of the system. Such devices are used to detect seismic vibrations produced by explosions, as well as by ground transportation equipment and other sources. No use of these devices to detect seismic signals generated by tornadoes has ever been reported.